FIG. 4 shows a combustion apparatus conventionally utilized as a boiler wherein RDF is used as its fuel.
With reference to FIG. 4, the combustion apparatus comprises a fluidized-bed furnace 1, a cyclone 2 (dust collector) disposed downstream from the furnace 1 for separating a free-flowing medium and a combustion residue discharged from the furnace 1 from combustion gases and collecting these solids, and a medium-residue return channel 3 provided between the cyclone 2 and the fluidized-bed furnace 1 for returning the free-flowing medium and the combustion residue collected by the cyclone 2 to the furnace 1 therethrough.
The fluidized-bed furnace 1 forms a fluidized bed from sand, or like free-flowing medium respectfully with primary air and secondary air sent from an air preheater. A multiplicity of water tubes (not shown) for a boiler are arranged within the furnace 1 to cover the inner peripheral surface thereof. These water tubes communicate at their upper ends with a steam drum 6 via an unillustrated header. RDF is supplied from an unillustrated scale conveyor to a hopper 7 in portions of predetermined quantity and placed into the fluidized-bed furnace 1 from the hopper 7. Noncombustibles and free-flowing medium are drawn off from the lower end of the furnace 1 and separated by a separator 8 disposed below the furnace 1, and the noncombustibles are discharged from the system, while the medium is returned to the furnace 1 by a conveyor 9 and an elevator 10.
The medium-residue return channel 3 is bifurcated into two branches at an intermediate portion thereof. One of the branches, 3a, is provided at intermediate portions thereof with a flow control valve 11 and a heat recovery unit 12 which are arranged in this order downstream from the cyclone 2. A superheater 13 for the boiler is disposed in the heat recovery unit 12. Medium transport air is forced into the heat recovery unit 12. The forward end of the other branch 3b is opened to the fluidized-bed furnace 1 at a position above the position here the hopper 7 is opened to the furnace. The free-flowing medium transported by the conveyor 9 and the elevator 10 is admitted into the branch 3b at an intermediate portion thereof.
A heat recovery column 14 is disposed downstream from the cyclone 2. This column 14 has in its interior a first flue 15 for passing combustion gases, discharged from the cyclone 2, from above downward therethrough, and a second flue 16 in communication with the lower end of the first flue 15 for passing the combustion gases from below upward therethrough. The inner peripheral surface of the first flue 15 is covered with a multiplicity of boiler water tubes (not shown), the upper ends of which also communicate with the steam drum 6 via an unillustrated header. Arranged within the second flue 16 are a boiler economizer 17 for passing therethrough water forwarded from a deaerator, and two superheaters 18, 19 connected in series with the superheater 13 in the heat recovery unit 12 for passing steam sent from the steam drum 6. A water injector 20 for adjusting the temperature and pressure of the steam by injecting water is disposed between the two superheaters 18, 19 within the second flue 16, also between the lower superheater 19 in the flue 16 and the superheater 13 in the heat recovery unit 12.
A temperature reduction column 21 and a bag filter 22 are arranged downstream from the heat recovery column 14. The combustion gases passing through the heat recovery column 14 are further reduced in temperature by the heat reduction column 21. The bag filter 22 serves to collect hydrogen chloride, sulfur oxides, soot and dust from the combustion gases. Slaked lime, or like neutralizing agent, and a reaction assisting agent, are added to the combustion gases at a position upstream from the filter. After passing through the bag filter 22, the combustion gases are released into the atmosphere through a stack.
Fly ash is discharged from the lower ends of the heat recovery column 14, temperature reduction column 21 and bag filter 22 and sent to a fly ash treating unit (not shown).
With the combustion apparatus thus constructed, RDF is sent into the fluidized-bed furnace 1 by the hopper 7. In the furnace 1, the free-flowing medium is formed into a fluidized bed with primary air and secondary air, and RDF is burned in the fluidized bed. The unburned combustibles are almost completely burned until the combustion gases and the medium enter the cyclone 2 from the upper end of the furnace 1. The medium and combustion residue discharged from the furnace 1 are separated from the combustion gases and trapped in the cyclone 2, passed through the two branches 3a, 3b of the return channel 3 and returned to the furnace 1. While passing through the branch 3a having the heat recovery unit 12, the medium and the residue have their heat transferred to steam flowing through the superheater 13, whereby the steam is superheated. The temperature of the furnace 1 can be lowered by adjusting the amount of medium transport air to be forced in and the opening degree of the flow control valve 11 and thereby increasing the quantities of medium and residue to be passed through the branch 3a having the heat recovery unit 12. Conversely, the temperature of the furnace 1 can be raised by reducing the quantities of medium and residue to be passed through the branch 3a having the heat recovery unit 12.
The combustion gases flowing out of the cyclone 2 enter the heat recovery column 14, flow down the first flue 15 first and consequently have their heat transferred to boiler water in the water tubes providing the wall of the flue, whereby the boiler water in the water tubes is heated and evaporated, and the temperature of the combustion gases is lowered. The combustion gases then flow upward through the second flue 16 and have their heat transferred to the boiler water in the water tubes constituting the wall of the flue, to the steam in the two superheaters 19, 18 and to the water in the economizer 17, whereby the boiler water in the water tubes is heated and evaporated, the steam is superheated, and the water in the economizer 17 is preheated to result in a drop in the temperature of the combustion gases.
The combustion gases subsequently flow into the temperature reduction column 21 and have their temperature further reduced. Slaked lime, or like neutralizing agent, and a reaction assisting agent, are thereafter added to the combustion gases, and the resulting mixture is led into the bag filter 22, in which hydrogen chloride, sulfur oxides, soot and dust are removed. The gases separated off are then released into the atmosphere.
On the other hand, the water sent from the deaerator and preheated during passage through the economizer 17 is admitted into the steam drum 6 and thereafter further heated within the water tubes of the fluidized-bed furnace 1 and the heat recovery column 14 constituting a boiler water circulation circuit to become a steam-water mixture, which is then sent to the steam drum 6 again. The steam is separated off in the steam drum 6, passed through the three superheaters 18, 19, 13 in succession and superheated with the heat of the combustion gases during passage through the superheaters 18, 19 and with the heat of the free-flowing medium during passage through the superheater 13. The superheated steam is sent to a steam turbine.
However, the conventional combustion apparatus has the problem that almost complete combustion of RDF in the fluidized furnace 1 increases the internal temperature of the furnace 1 to a considerably high level and necessitates supply of secondary air at a high rate to result in the presence of a large amount of oxygen, consequently producing large quantities of NOx with the nitrogen contained in RDF and the nitrogen afforded by the air. Further, since the combustion gases flowing into the cyclone 2 have a considerably high temperature, for example, of at least about 800.degree. C., the apparatus has another problem that the ash resulting from incineration and contained in the gases is partly melted and adheres to the cyclone 2 to entail an impaired medium separation efficiency. Another problem is also encountered in that the molten ash adheres to the free-flowing medium, causing faulty fluidization in the furnace 1. An attempt to reduce the temperature of the furnace 1 to preclude these problems will entail the problem of giving rise to incomplete combustion to produce increased amounts of unburned combustibles.
An object of the present invention is to overcome the foregoing problems and to provide a combustion apparatus which is diminished in the quantities of NOx produced, further permitting a cyclone, or like dust collector, to achieve an improved efficiency in separating off a free-flowing medium and assuring a fluidized-bed furnace of effective fluidization.